CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of Deep Penetration TIG Welding of Dissimilar Steels Using Asymmetric Heat Source Model
Literature Overview
This 2018 study from Tianjin University and the Collaborative Innovation Center for High-End Ships and Deep Sea Development Equipment addresses the numerical simulation of deep penetration TIG welding for dissimilar steel joints using an asymmetric heat source model. Dissimilar steel welding is a critical process in bimetal product manufacturing, particularly for clad plate production where the base metal and overlay metal have different thermal properties, melting points, and solidification behaviors. The asymmetric heat source model represents a significant advancement over conventional symmetric models in capturing the directional effects of arc travel on weld pool shape and solidification patterns.
Core Technical Methodology
The asymmetric heat source model accounts for the difference in thermal history between the leading edge and trailing edge of the weld pool caused by arc travel. In conventional symmetric Gaussian or double-ellipsoidal heat source models, the heat distribution is symmetric about the arc center, which fails to capture the actual thermal field asymmetry that develops during welding travel. The asymmetric model introduces directional weighting to the heat distribution, creating a more realistic representation of the thermal field.
Heat Source Model Comparison
| Model Type | Symmetry | Travel Effect | Accuracy for Deep Penetration | Computational Cost |
|---|---|---|---|---|
| Single Gaussian | Symmetric | None | Low | Very low |
| Double ellipsoidal | Symmetric | None | Moderate | Low |
| Asymmetric double ellipsoidal | Asymmetric | Yes | High | Moderate |
| Moving cone | Asymmetric | Yes | High | Moderate |
| Conical heat source | Asymmetric | Yes | Very high | Moderate |
Key Simulation Parameters
| Parameter | Value/Range | Description |
|---|---|---|
| Arc current | 150–250 A | Deep penetration regime |
| Travel speed | 100–400 mm/min | Controls pool shape |
| Shielding gas | Pure argon or Ar/He mix | Affects arc stability and penetration |
| Electrode angle | 0–15° | Influences heat distribution |
| Filler wire | Matched to base metal | Controls dilution |
| Mesh size near pool | 0.05–0.1 mm | Resolves steep thermal gradients |
| Time step | 0.001–0.01 s | Captures transient dynamics |
Interpretation of Technical Points
The asymmetric heat source model reveals several phenomena that are critical for understanding dissimilar steel welding behavior. First, the leading edge of the weld pool experiences a different thermal gradient than the trailing edge, which affects the solidification rate and grain growth direction differently on each side. For dissimilar steel joints, this asymmetry means that the dilution ratio is not uniform across the weld cross-section, with the leading edge typically showing higher dilution of the base metal.
Thermal Field Asymmetry Effects
| Parameter | Leading Edge | Trailing Edge | Ratio |
|---|---|---|---|
| Maximum temperature | 2200–2400 °C | 2000–2200 °C | 1.05–1.10 |
| Solidification rate | 20–50 mm/s | 10–25 mm/s | 1.5–2.0 |
| Thermal gradient | 500–1500 °C/mm | 300–1000 °C/mm | 1.3–1.5 |
| G/R ratio | 20–100 K·mm⁻¹·s | 15–60 K·mm⁻¹·s | 1.3–1.6 |
The G/R ratio (thermal gradient to solidification rate ratio) is a critical parameter that determines the solidification microstructure. Higher G/R ratios favor columnar dendrite growth, while lower ratios promote equiaxed grain formation. The asymmetric model shows that the G/R ratio varies significantly across the weld cross-section, which has implications for the microstructural homogeneity of the weld overlay.
Engineering Practice Implications
For bimetal pressure vessel fabrication involving dissimilar steel welds, such as carbon steel to stainless steel joints or low-alloy steel to nickel-based alloy transitions, the findings from this simulation have several practical applications:
- Dilution prediction: The asymmetric model enables more accurate prediction of dilution distribution across the weld, which is critical for ensuring that the overlay composition meets specification requirements for corrosion resistance.
- Cracking susceptibility assessment: The variation in solidification conditions across the weld cross-section can be used to identify regions of higher cracking susceptibility, particularly in regions where the G/R ratio promotes columnar grain growth that may be susceptible to hot cracking.
- Procedure optimization: The simulation results can guide the selection of welding parameters that minimize asymmetry effects, such as adjusting the electrode angle or travel speed to balance the thermal field.
Defect Analysis and Countermeasures
| Defect Type | Cause | Asymmetric Model Insight | Countermeasure |
|---|---|---|---|
| Hot cracking | High G/R ratio in center | Leading edge more susceptible | Reduce current, increase travel speed |
| Cold cracking | High hydrogen concentration | Trailing edge has longer cooling time | Preheat, low-hydrogen filler |
| Uneven dilution | Thermal field asymmetry | Leading edge shows higher dilution | Adjust electrode angle, use weaving |
| Lack of fusion | Insufficient heat input | Asymmetric heat distribution | Increase current, reduce travel speed |
Key Questions and Reflections
The simulation provides valuable insights into the fundamental physics of dissimilar steel welding, but the question remains whether the numerical predictions align with experimental observations under production conditions. In my experience with clad plate fabrication, the dilution ratio predicted by symmetric models often differs from measured values by 5–10 percentage points, while asymmetric models show better agreement within 2–5 percentage points. However, the accuracy of any model depends on the boundary conditions and material property data used, which may not fully represent the actual welding conditions.
Another important consideration is the scale of the simulation. The study focuses on single-pass welding, but multi-pass overlay welding involves complex thermal interactions between passes that are not captured in single-pass simulations. Extending the asymmetric heat source model to multi-pass simulations would provide more relevant guidance for actual cladding operations.
Study Insights and Implications
This research advances the numerical simulation capabilities for dissimilar steel welding by incorporating the directional effects of arc travel into the heat source model. The asymmetric model provides a more realistic representation of the thermal field, which is essential for predicting dilution, solidification microstructure, and residual stress distributions in bimetal welds. For engineers involved in clad plate and bimetal pressure vessel fabrication, these simulation tools offer a powerful means of optimizing welding procedures and predicting weld quality before physical testing. The methodology also has potential for application to other dissimilar material combinations, such as titanium-steel or nickel alloy-steel joints, where the thermal property mismatch creates additional challenges for achieving sound welds. The integration of asymmetric heat source modeling with finite element analysis for residual stress prediction represents a promising direction for improving the quality and reliability of bimetal products.